Solid electrolyte thin film and method for producing the same

ABSTRACT

The present invention relates to a solid electrolyte thin film and a manufacturing method for the same. The solid electrolyte thin film of the present invention comprises yttrium stabilized zirconia and is characterized in that the thickness of the film is 10 to 50 μm. In an embodiment, 80% or more of the surface area of the film is occupied by crystals having a crystal grain size of 6 to 18 μm. The present invention also includes a method for producing a solid electrolyte thin film comprising a sintered body of solid electrolyte powders. The method includes regulating the grain size of said solid electrolyte powders in the range of 0.1 to 5.0 μm; preparing a slurry solvent by mixing a binder and other materials with a solvent containing 10 to 80 wt % of a low volatile solvent, and mixing the slurry solvent with the solid electrolyte powder; and coating the obtained slurry onto a substrate. The slurry composition is prepared so that it has a viscosity in the range of 1 to 500 cps.

FIELD OF THE INVENTION

The present invention relates to a dense solid electrolyte thin filmutilized in solid electrolyte type fuel cells (hereafter denoted SOFC),oxygen sensors, oxygen pumps, and the like and a method for producingthe same.

PRIOR ART

Solid electrolyte thin films having permeability to oxygen ions (O²⁻) aswell as gas tightness are used in SOFC and such. These solid electrolytethin films (made of ZrO₂, CeO₂, and such) are required to be thin anddense to achieve these characteristics. In addition, they are requiredto be economically formed into large sized thin films.

The conventional art technique is described herein with reference, as anexample, to solid electrolyte thin films for SOFC. For SOFC, in general,an air electrode (made of LaMnO₃, and such materials) having a thicknessof 30˜2000 μm is formed on a porous substrate having a thickness of0.3˜5.00 mm, and a solid electrolyte thin film having a thickness of30˜1000 μm is formed over the air electrode. Over the solid electrolytethin film, a fuel electrode (made of Ni base cermets, and suchmaterials) is formed.

In order to obtain solid electrolyte thin films which are thin anddense, and at the same time, low in production cost and excellent inmass productivity as a goal, the following have been proposed in thepast:

1) Production Process by Plasma Spray Coatings (Patent publicationnumber Kokai sho: 61-198570):

This production process is characterized in that solid electrolytestarting materials comprising cerium-oxides or zirconium-oxides anddivalent or trivalent metal-oxides of alkaline earth metals or rareearth elements and the like are formed into a solid solution. Then, thestarting material in the form of a solid solution are crushed, and thegrain size of the obtained powder is regulated, and then the powder iscoated as an electrolyte film on a fuel cell substrate by plasma spraycoating. According to an example described in the publishedspecification, a solid electrolyte thin film having a thickness of 200μm and terminal voltage of 790 mv is obtained by using plasma spraypowder having a grain size of 2 μm or less.

2) Production Process by CVD.EVD (Chemical Electrical vapor deposition)(Patent publication number Kokai sho: 61-91880):

This production process is characterized in that the first electrode isadhered onto a porous support member, an intermediate layer substancewith electrical conductivity and oxygen permeability is adhered onto thefirst electrode to protect the first electrode from a high temperaturevapor of halides, the intermediate layer substance is contacted with ahigh temperature vapor of metal halide to form a solid electrolytecomposed of metal oxides on the whole surface of the intermediate layer,and the second electrode is adhered on the whole surface of the solidelectrolyte.

3) Production Process by Slurry Coating (Patent publication number KokaiHei:1-93065)

This production process is a process for manufacturing acylindrically-shaped solid electrolyte wherein a fuel electrode layerand an air electrode layer are overlapped and an electrolyte layer isinterposed there between, and characterized in that either one of theair electrode layer or the fuel electrode layer is formed into a tubularshape. A powder slurry of each material for forming the electrolyte andthe other electrode layer is coated on the surface of the tube one byone and dried, and then the tube is baked. According to an example ofthe published specification, an yttrium stabilized zirconia (YSZ) thinfilm having a thickness of 150 μm is obtained.

(4) Production by Plasma Spray Coating and Filler Slurry Coating (Patentpublication number Kokai Hei:2-220361)

This production process relates to a tubular type SOFC formed by layingup a fuel electrode, a solid electrolyte and an air electrode on theoutside of a substrate tube, and is characterized in that a fillermaterial containing 40 wt % or more (solids concentration) of yttriumstabilized zirconia is coated on openings of laid-up solid electrolyteon the substrate tube, and then the tube is dried and baked. Accordingto an example described in the published specification, a slurrycontaining YSZ powders with a grain size of 0.05 to 2.5 μm is coated(brush coating) on an air plasma spray coated film having a thickness of100 μm, and then the film is dried and baked. Finally, a solidelectrolyte thin film with very low gas permeability was obtained.

TASKS TO BE SOLVED BY THE INVENTION

The aforesaid conventionally proposed techniques involve the followingproblems:

1) Plasma Spray Coating Process:

The films gained through this process is fundamentally porous.Therefore, the film should be made relatively thick to extinguish gaspermeability, or a filler should be applied to seal porosities afterplasma spray coating (like the above-described process in item (4)).

2) CVD Processes, EVD Process:

These processes are appropriate for forming dense thin films. However,they require expensive equipment since film forming should be carriedout under a special atmosphere and a physical condition isolated fromthe air. For large sized parts, large sized equipment is naturallyrequired to accommodate the parts. Accordingly, film coating onto largeparts is difficult as well as low in productivity and high in cost.

3) Slurry Coating Process:

This process is an economical process since film formation is carriedout in the air and expensive equipment is not necessary. This process,however, has been understood to have problems in density and thicknessof the film, and the solid electrolyte thin film disclosed as an examplein the Patent publication number Hei 1-93065 has a thickness of 150 μm,which is considerably thicker than the target thickness of 10 to 50 μmfor this kind of a film.

4) Plasma Spray Coating and Slurry Filling Process:

This process requires two-step work, and moreover the film thicknesstends to be thicker.

It is an object of the present invention to provide a method forproducing a dense solid electrolyte thin film having a thickness in therange of 10 to several hundred μm, which is economical and capable ofbeing mass produced and which is applicable to a large surface area.

It is a further object of the present invention to provide a solidelectrolyte thin film that can be manufactured through a slurry coatingprocess to be a dense and thin film.

MEANS FOR SOLVING THE TASKS AND ACTION

In order to achieve the above tasks, the solid electrolyte thin film ofthe present invention is a solid electrolyte thin film comprisingstabilized ZrO₂ containing 3 to 20 mol % of Y₂ O₃ (YSZ); and ischaracterized in that the thickness of the film is 10 to 50 μm and morethan 80% of the whole surface area of the film surface is occupied bycrystals having a crystal grain size of 6 to 18 μm.

By controlling the film thickness and the crystal grain size asindicated above, a dense solid electrolyte thin film having almost nocracks among the crystal grains can be obtained.

The solid electrolyte thin film of the present invention may also be asolid electrolyte thin film comprising stabilized ZrO₂ containing 3 to20 mol % of Y₂ O₃ (YSZ) and, having a thickness of 10 to 50 μm, and ahalf value width in X-ray diffraction (2θ) of less than 0.16.

When the half value thickness is within the above range, since thecrystal characteristics are excellent and uniform sintering is carriedout, a dense solid electrolyte thin film without cracks among crystalgrains and without open porosities can be obtained.

The method of the present invention is a method for producing a thinfilm comprising a sintered body of solid electrolyte powder,characterized in that the method includes a grain size regulatingprocess wherein the size of said solid electrolyte powders is regulatedwithin 0.1 to 5.0 μm; a slurry preparing process wherein a slurrysolvent is prepared by mixing 0.1 to 10 parts of a binder, 0.1 to 4parts of a dispersant and 0.1 to 4 parts of an anti-foaming agent to 100parts of a solvent containing 10 to 80 wt % of a low-volatile solventand this slurry solvent is mixed with 5 to 40 parts by weight of saidsolid electrolyte powders whose grain size has been regulated asindicated above, to obtain a slurry; and a coating process wherein theobtained slurry is coated onto a substrate, the slurry composition beingprepared so as to arrange the viscosity of said slurry in the range of 1to 500 cps.

The solid electrolyte used in the production method of the presentinvention is not limited to a specific substance. Y₂ O₃ stabilized ZrO₂(YSZ), for example, may be applicable. However, in any kinds ofelectrolytes, a material containing less impurity is more preferable;particularly, an amount of the total impurities such as Si, Mn and thelike is preferably 0.1 wt % or less. The Y₂ O₃ content in YSZ ispreferably 3 to 20 mol %, more preferably 8 to 12 mol %, since ionconductivity is excellent in this range.

The grain size of the solid electrolyte powders is preferably between0.1 and 5.0 μm, more preferably 0.2 to 0.4 μm. When a material with arelatively wide range of grain size of 0.1 to 5 μm is used, the filmthickness for obtaining a dense (gas-tight) films is required to be fromseveral tens to several hundred μm. However, when materials with arelatively narrow range of grain size of 0.2 to 0.4 μm is used, densefilms may be formed by a film thickness of from several μm to severaltens μm. The materials are preferably regulated in the grain size byusing classifiers such as pneumatic classifiers, and the like.

The characteristic feature of the solvent to be used in the slurrysolvent of the present invention resides in that the solvent contains 10to 80 wt %, more preferably 15 to 40 wt %, of a low-volatile solvent.This low-volatile solvent acts to suppress altering of the slurryviscosity during preparation and storage process and to prevent crackgeneration caused by drying after film formation using the slurry (forexample, after forming a film by dipping). Here, the degree ofvolatility is preferably below 1, compared with that of butyl acetate as100.

An example of a low-volatile solvent is α-terpineol. The reason that thecontent of low-volatile solvent is limited to 10 to 80 wt % is that thefilm is susceptible to cracks in the process of drying after the filmformation (for example, by dipping) in case of a low content (less than10 wt %), and that dispersibility of the powder becomes poor in case ofa high content (over 80 wt %). The content of the low-volatile solventis more preferably 15 to 40 wt %. Within this range, dispersion ofpowders in the slurry and drying condition after dipping is bestbalanced in characteristics.

Solvents other than the low-volatile solvent contained in the slurrysolvent act to improve dispersibility of powder and defoambility. Ethylalcohol is preferable for such a solvent.

Binders contained in the slurry solvent of the present invention act toimprove coating performance (adhesion) of powders to the substrate. Thereason why the amount of binders is limited to 0.1 to 10 parts based on100 parts of the solvent is that coating performance is poor in case ofa low content (less than 0.1 wt %), and that dispersibility of powderbecomes poor in case of a high content (over 10 wt %) Ethyl cellulose,for example, is preferable for such a binder.

Dispersants contained in the slurry of the present invention act toimprove dispersibility of powder. The reason why the amount ofdispersant is limited to 0.1 to 4 parts based on 100 parts of thesolvent is that dispersibility is poor in case of a low content (lessthan 0.1 wt %) and that the slurry tends to denaturate in case of a highcontent (over 4 wt %). Polyoxyethylene alkyl phosphoric ester, forexample, is preferable as a dispersant.

Anti-foaming agents contained in the slurry solvent of the presentinvention act to eliminate foams in the slurry. The reason why theamount of anti-foaming agents is limited to 0.1 to 4 parts based on 100parts solvent is that the effect is not expected in case of less thanthe above range and binders in the slurry tends to denaturate in case ofover the above range. Sorbitan sesqui-oleate, for instance, ispreferable for an anti-foaming agent.

For mixing the powders, any common process, such as the use of ballmills may be used.

The process for coating slurry on the substrate in the method of thepresent invention is not narrowly limited to a certain process. It maybe accomplished by dipping, spraying, brushing, or such processes. Ofthese, dipping is preferable since it is simple, highly mass-productive,and low in cost. As a dipping method, dipping under pressured gases orvacuum may be applicable in addition to a normal dipping method in whichthe substrate is dipped in a slurry under air. Times of dipping may beselected according to the required film thickness and the slurrycompositions.

It is important in the production method of the present invention tocontrol slurry viscosity within the range of 1 to 500 cps, morepreferably 1 to 100 cps. The reason why the low limit is restricted to 1cps is to prevent the slurry from penetrating into the porous substrate.The reason why the upper limit is restricted to 500 cps is to preventcracks during drying process after coating slurry.

In order to control the slurry viscosity within the above range,contents of binders and powders are controlled within the rangedescribed above.

After dipping in the slurry, drying and baking may be carried out in anyapplicable manner.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows SEM photographs showing the surface and sectional crystalstructure of the thin film of an example of the present invention and acomparative example. (A) shows a dense film of the example of thepresent invention and (B) shows a porous film of the comparativeexample.

EXAMPLES

Examples of the present invention and comparative examples are nowdescribed.

Table 1 shows a list of examples and comparative examples to bedescribed below.

                  TABLE 1                                                         ______________________________________                                                             α-terpineol/                                         ZrO.sub.2  ethyl ethyl ZrO.sub.2                                              powder  alcohol cellulose powder                                              parts by Viscosity parts by parts by grain size                               weight cps weight weight μm                                              ______________________________________                                        Comparative                                                                            4      1 or less                                                                              8.5/91.5                                                                               0.08  0.3 μm                               Ex. 1     (0.2-0.4)                                                           Ex. 1  5 1 ˜ 5 10/90 0.1 0.3 μm                                           (0.2-0.4)                                                                Ex. 2 25 20 ˜ 30 50/50 2.0 0.3 μm                                         (0.2-0.4)                                                                Ex. 3 40 450 ˜ 500 80/20 4.0 0.3 μm                                       (0.2-0.4)                                                                Comparative 45 650 or 85/15 4.2 0.3 μm                                     Ex. 2  more   (0.2 ˜ 0.4)                                               Comparative 15 10 ˜ 20 50/50 1.0 0.1 μm or                           Ex. 3     less                                                                Ex. 4 25  60 ˜ 140 " 2.8 2.0                                                 (0.1-5.0)                                                                Comparative 25 100 ˜ 200 " 4.0 8                                        Ex. 4     (5-10)                                                            ______________________________________                                    

Comparative Example 1, present-invention Examples 1 to 3, andComparative Example 2 are experiments carried out by fundamentallyvarying slurry viscosity. Comparative Example 3, present-inventionExample 4 and Comparative Example 4 are experiments on varying ZrO₂powder grain size.

Comparative Example 1

(1) Solid electrolyte powder preparation:

ZrO₂ +8 mol % Y₂ O₃ powders were ground by using a ball mill and thenthe grain size was regulated to the range of 0.2 to 0.4 μm (average: 0.3μm) by a pneumatic classifier.

(2) Slurry solvent:

8.5 parts of α-terpineol as a low-volatile solvent and 91.5 parts ofethyl alcohol as a volatile solvent were mixed and then 0.08 parts ofethyl cellulose as a binder, 1 part of polyoxyethylene alkyl phosphoricester as a dispersant and 1 part of sorbitan sesqui-oleate were added tothe solvent and mixed to obtain the slurry solvent.

(3) Slurry preparation:

100 parts by weight of the above slurry solvent and 4 parts by weight ofthe above powders having the above-described grain size were mixed toobtain a slurry with a viscosity of less than 1 cps.

(4) Dipping:

The slurry was coated onto the surface of the substrate (substance:LaSrMnO₃ size: 12 mm diameter×10 mm diameter×100 mm length, porosity:35%) by dipping.

(5) Drying:

The substrate was held at room temperature for 1 hour and then at 100°C. for 1 hour.

(6) Baking:

The substrate was baked at 1500° for 5 hours. The film obtained throughthe above processes had a thickness of less than 10 μm and a gaspermeability coefficient (N₂ gas used) of 1.0×10⁻¹⁹ m² ·s/kg or more.Consequently a dense film was not obtained.

Example 1

The process of Comparative Example 1 was followed except for changingthe slurry solvent and the slurry preparation as follows:

(1) Slurry solvent:

α-Terpineol 10 parts, ethyl alcohol 90 parts, ethyl cellulose 0.1 parts,polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part.

(2) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (5parts), having a viscosity of 1 to 5 cps.

The solid electrolyte thin film obtained through the above processes hada thickness of about 10 μm and a gas permeability coefficient of lessthan 8.5×10⁻¹¹ m² ·s/kg. Consequently a considerably dense film could beobtained. Such dense films can be used for fuel electrodes, and thelike.

Example 2

The process of Comparative Example 1 was followed except for changingthe slurry solvent and the slurry preparation, as follows:

(1) Slurry solvent:

α-Terpineol 50 parts, ethyl alcohol 50 parts, ethyl cellulose 1.8 parts,Polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part

(2) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (25parts), having a viscosity of 20 to 30 cps

The solid electrolyte film obtained through the above processes had athickness of 15 to 25 μm and a gas permeability coefficient of less than8.5×10⁻¹¹ m² ·s/kg. Thus, a considerably dense film was obtained. Suchdense films can be used for fuel electrodes, and the like.

Example 3

The process of Comparative Example 1 was followed except for changingthe slurry solvent and the slurry preparation, as follows:

(1) Slurry solvent:

α-Terpineol 80 parts, ethyl alcohol 20 parts, ethyl cellulose 4 parts,polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part.

(2) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts), was mixed with ZrO₂ powder (40parts), having a viscosity of 450 to 500 cps.

The solid electrolyte thin film obtained through the above processes hada thickness of 40 μm. The gas permeability of this film was measured byusing a nitrogen gas and found to be zero, and, thus, or dense solidelectrolyte thin film was obtained.

Comparative Example 2

The process of Comparative Example 1 was followed except for changingthe slurry solvent and the slurry preparation, as follows:

(1) Slurry solvent:

α-Terpineol 85 parts, ethyl alcohol 15 parts, ethyl cellulose 4.2 parts,polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part.

(2) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (45parts), having a viscosity of 650 cps or more.

In this process, after drying subsequent to the dipping process, a fewcracks were observed in the thin film. These cracks propagated duringthe baking process, and, therefore, a good solid electrolyte thin filmwas not obtained.

Comparative Example 3

The process of Comparative Example 1 was followed except for changingthe grain size of the solid electrolyte powders, the slurry solvent andthe slurry preparation, as follows:

(1) Size of the solid electrolyte powder:

less than 0.1 μm.

(2) Slurry solvent:

α-Terpineol 50 parts, ethyl alcohol 50 parts, ethyl cellulose 1 parts,polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part.

(3) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (15parts), having a viscosity of 5 to 10 cps.

As a result of the above processes, a solid electrolyte thin film havinga thickness of about 10 to 15 μm was obtained. The gas permeabilitycoefficient of this film was 2.4×10⁻¹⁰ m² ·s/kg or more and this filmwas found to be rather porous. Consequently, a good solid electrolytethin film was not obtained.

Example 4

The process of Comparative Example 1 was followed except for changingthe grain size of the solid electrolyte powders, the slurry solvent andthe slurry preparation, as follows:

(1) Size of the solid electrolyte powder:

0.1 to 5 μm (average grain size 2 μn).

(2) Slurry solvent:

α-Terpineol 50 parts, ethyl alcohol 50 parts, ethyl cellulose 2.8 parts,polyoxyethylene alkyl phosphoric ester 1 part, sorbitan sesqui-oleate 1part.

(3) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (25parts), having a viscosity of 60 to 140 cps.

As a result of the above process, a dense solid electrolyte thin filmhaving a thickness of 20 to 35 μm and a gas permeability coefficient ofless than 9.0×10⁻¹¹ m² ·s/kg was obtained.

Comparative Example 4

The process of Comparative Example 1 was followed except for changingthe grain size of the solid electrolyte powders, the slurry solvent andthe slurry preparation, as follows:

(1) Size of the solid electrolyte powder:

5 to 10 μm (average grain size 8 μm).

(2) Slurry solvent:

α-Terpineol parts 50 parts, ethyl alcohol 50 parts, ethyl cellulose 4parts, polyoxyethylene alkyl phosphoric ester 1 part, sorbitansesqui-oleate 1 part.

(3) Slurry preparation, composition and viscosity:

the above slurry solvent (100 parts) was mixed with ZrO₂ powder (25parts), having a viscosity 10 to 200 cps.

As a result of the above processes, a solid electrolyte thin film havinga thickness of about 25 to 40 μm was obtained. The gas permeabilitycoefficient of this film was 1×10⁻⁹ m² ·s/kg or more, and this film wasfound to be rather porous. Thus, good solid electrolyte thin film wasnot obtained.

Crystal Grain Size

SEM observation was conducted on the film structures of the solidelectrolyte thin films of Examples 1, 2 and 3 where dense films wereobtained, and on the solid electrolyte thin films of ComparativeExamples 3 and 4, where only porous films were obtained. The dense filmsof Examples 1-3 were found to be composed of crystals having 6 to 18 μmcrystal grain size, and the primary grain size of the porous films ofComparative Examples 3 and 4 were found to be 1 to 5 μm. The reason forthis is that, in the examples of the present invention, diffusionabilityand sinterability between crystal grains were improved whereby crystalgrains could grow. In contrast, in the Comparative Examples, thediffusionability and sinterability between crystal grains were low andthe grains did not grow.

Accordingly, it may be concluded that a dense solid electrolyte thinfilm is obtainable by controlling crystal grain size within the range of6 to 18 μm.

FIG. 1 shows SEM photographs showing surface and sectional crystalstructure of the solid electrolyte thin film of an example of thepresent invention and a comparative example. (A) shows a dense film ofthe example of the present invention and (B) shows a porous film of thecomparative example.

X-RAY DIFFRACTION HALF VALUE WIDTH

The full width at half maximum (FWHM) by X-ray diffraction was measuredon each of the solid electrolyte thin films of Examples 1, 2 and 3,where dense films were obtained, and on the solid electrolyte thin filmsof Comparative Examples 3 and 4, where only porous films were obtained.The FWHMs were measured based on the peak where 2θ=30.2 to 30.4, whichis the main peak of the 8 mol % Y₂ O₃ stabilized ZrO₂. MXP-18manufactured by Mack Sience was used for a diffraction equipment andradiation source was Cu, tube voltage was 50·OKV and tube current was300 mA.

The FWHMs were within the range of 0.14 to 0.16 in dense films of theexamples of the present invention, and 0.18 to 0.20 in porous films ofthe comparative examples.

Another example of the present invention is now described.

(1) Solid Electrolyte Powder Preparation:

ZrO₂ +8 mol % Y₂ O₃ powders were ground by a ball mill and then thegrain size was arranged by a pneumatic classifier.

(2) Slurry Solvent:

40 parts of α-Terpineol and 60 parts of ethyl alcohol were mixed andthen 4 parts of ethyl cellulose was added as a binder, 1 part ofpolyoxyethylene alkyl phosphoric ester was added as a dispersant and 1part of sorbitan sesqui-oleate was also added, and all materials mixedto obtain the slurry solvent.

(3) Slurry Preparation:

100 parts by weight of the above-described slurry solvent and 80 partsof the above-described powders arranged were mixed to obtain a slurrywith a viscosity of 300 to 350 cps.

(4) Dipping:

The slurry was coated onto a surface of the substrate (substance:LaSrMnO₃ size: 12 mm diameter×10 mm diameter×100 mm length, porosity:35%) by dipping.

(5) Drying:

The substrate was held at room temperature for 1 hour and then at 100°C. for 1 hour.

(6) Baking:

The substrate was baked at 1500° for 5 hours.

As a result of the above process, a solid electrolyte thin film having athickness of 30 μm was obtained. The gas permeability of the film wasmeasured but no gas permeability was admitted. Thus, a dense electrolytefilm was obtained.

ADVANTAGEOUS EFFECT OF THE INVENTION

As clearly understood from the above descriptions, the present inventionexhibits the--following advantageous effects.

(1) A solid electrolyte thin film can be formed on the whole surface orany part of variously shaped substrates (flat plate or inner and outersurfaces of pipes, and the like.)

(2) Dense and thin films can be formed.

(3) In comparison with CVD•EVD methods or plasma spray coating methods,expensive manufacturing equipment is not required, and application tolarge scale parts is easy.

(4) According to (1) to (3), SOFC cells with high performance and lowproduction cost and elements for example, oxygen sensors and oxygenpumps, and the like, can be provided.

We claim:
 1. A solid electrolyte film comprising stabilized ZrO₂, saidZrO² comprising 3 to 20 mol % Y₂ O₃, wherein said solid electrolyte filmhas a thickness of between 10 μm and 50 μm, and at least 80% of thesurface area of said film is occupied by crystals having a crystal grainsize of between 6 μm and 18 μm.
 2. A solid electrolyte thin filmcomprising stabilized ZrO₂, said ZrO₂ comprising 3 to 20 mol % Y₂ O₃,wherein said solid electrolyte film has a thickness of between 10 μm and50 μm, and said solid electrolyte film has a FWHM (2θ), as measured byan X-ray diffraction technique, of 0.16 or less.
 3. A solid electrolytefilm according to claim 1 wherein said solid electrolyte film has a gaspermeability coefficient of 9×10⁻¹¹ m²· s/kg or less.
 4. A solidelectrolyte thin film according to claim 2 wherein said solidelectrolyte film has a gas permeability coefficient of 9×10⁻¹¹ m²· s/kgor less.
 5. A method for producing a solid electrolyte film comprising asintered body of solid electrolyte powders, said method comprising thesteps of:(A) regulating the grain size of said solid electrolyte powdersin the range of 0.1 to 5.0 μm; (B) preparing a slurry having a viscosityof between 1 cps and 500 cps, by mixing together:(1) a slurry solventcomprising:a) 0.1 to 10 parts by weight of a binder; b) 0.1 to 4 partsby weight of a dispersant; c) 0.1 to 4 parts by weight of ananti-foaming agent; and d) 100 parts by weight of a solvent containingbetween 10% and 80% by weight of low volatile solvent; and (2) 5 to 40parts by weight of said solid electrolyte powder; and (C) coating saidslurry onto a substrate.
 6. The method according to claim 5, whereinsaid solid electrolyte comprises a stabilized ZrO₂ comprising 3 to 20mol % Y₂ O₃.
 7. The method according to claim 5 wherein said solidelectrolyte powder has a grain size of between 0.2 μm and 0.4 μm.
 8. Themethod according to claim 6 wherein said solid electrolyte powder has agrain size of between 0.2 μm and 0.4 μm.